278 The Americaii Geologist. May, i899 
grained, greenish grey massive rock with a conchoidal fracture. 
It weathers a reddish brown with a slightly roughened surface 
owing to the decay of the olivine. 
Specimens, which were subsequently sectioned, were taken 
from exposures at four different localities. Each of these lo- 
calities is represented by three specimens, of which two are 
from the borders of the dike and one from the centre. These 
slides show the rock to be an olivine diabase of a normal char- 
acter. Plagioclase, pyroxene, and olivine constitute the chief 
mass of the rock, while the structure is typically ophitic. 
The two chief constituents, pyroxene and plagioclase, are 
about equally abundant. The pyroxene is augite with the 
usual characters: a pinkish grey, non-pleochroic mineral with 
the characteristic birefringence, cleavage and twinning (parallel 
to 010). It always occurs in allotriomorphic areas which are 
often so collected in groups as to give a porphyritic cast to the 
rock. Near the edge of the dike where the mass of the rock 
becomes aphanitic these areas become more decidedly pheno- 
crystal. Sometimes large areas show a curved cleavage and 
undulatory extinction. The augite contains no inclusions and 
is comparatively fresh but shows some slight alteration to 
chlorite. 
The other ferromagnesian constituent is distinguished from 
augite by its colorlessness and its idiomorphic and columnar 
character. Besides the longitudinal cleavage (010) there is ir- 
regularly developed a conspicuous transverse parting parallel 
to the base. The index of refraction is high, but not equal to 
that of the augite. Most of the columnar sections show the 
emergence of an axis while the other sections show high 
double refraction. The extinction is parallel except in unsym- 
metrical sections. This olivine so closely resembles the color- 
less or white pyroxene noted in the diabase of the New Haven 
region, that only the results of optical tests convinced the writ- 
er that the constituent was olivine and not a variety of pyrox- 
ene. Considering the readiness with which olivine yields to al- 
teration, the mineral is very fresh, though more altered than 
the augite. The alteration product is a yellow serpentine. 
Plagioclase appears in two generations: as a net work of 
intergrown lath-shaped crystals of varying lengths and 
